Helium is a scarce element on earth and its numerous scientific and industrial applications continue to drive a growing demand. For example, common uses of gas-phase helium include welding, lifting (balloons), and semiconductor and fiber optic manufacturing. In the liquid phase, common uses include refrigeration of certain medical and scientific equipment, purging fuel tanks (NASA), and basic research in solid-state physics, magnetism, and a wide variety of other research topics. Because of the widespread utility of helium, its limited availability, and the finite reserves of helium, it is considered a high-cost non-renewable resource. Accordingly, there is an increasing interest in recycling helium and similar noble gases.
In particular, liquid helium is used as the refrigerant in many applications in which it is necessary to reach temperatures below −200° C. Such applications are frequently related to the use of superconductors, and particularly in low-temperature physics research equipment which operates in evacuated and insulated containers or vacuum flasks called Dewars or cryostats. Such cryostats contain a mixture of both the gas and liquid phases and, upon evaporation, the gaseous phase is often released to the atmosphere. Therefore it is often necessary to purchase additional helium from an external source to continue the operation of the equipment in the cryostat.
One of liquid helium's most important applications is to refrigerate the high magnetic field superconducting coils used in magnetic resonance imaging (MRI) equipment, which provides an important diagnostic technique by non-invasively creating images of the internal body for diagnosing a wide variety of medical conditions in human beings.
The largest users of liquid helium are large international scientific facilities or installations, such as the Large Hadron Collider at the CERN international laboratory. Laboratories such as CERN recover, purify, and re-liquefy the recovered gas through their own large scale (Class L) industrial liquefaction plants, which typically produce more than 100 liters/h and require input power of more than 100 kW. For laboratories with more moderate consumption, medium (Class M) liquefaction plants are available that produce about 15 liters/hour. These large and medium liquefaction plants achieve a performance, R, of about 1 liter/hour/kW (24 liters/day/kW) when the gas is pre-cooled with liquid nitrogen, and about 0.5 liters/hour/kW (12 liters/day/kW) without pre-cooling.
For smaller scale applications small-scale refrigerators are now commercially available which are capable of achieving sufficiently low temperatures to liquefy a variety of gases and, in particular, to liquefy helium at cryogenic temperatures below 4.2 Kelvin. In the industry, these small-scale refrigerators are normally referred to as closed-cycle cryocoolers. These cryocoolers have three components: (1) a coldhead (a portion of which is called the “cold finger” and typically has one or two cooling stages), where the coldest end of the cold finger achieves very low temperatures by means of the cyclical compression and expansion of helium gas; (2) a helium compressor which provides high pressure helium gas to and accepts lower pressure helium gas from the coldhead; and (3) the high and low pressure connecting hoses which connect the coldhead to the helium compressor. Each of the one or more cooling stages of the cold finger has a different diameter to accommodate variations in the properties of the helium fluid at various temperatures. Each stage of the cold finger comprises an internal regenerator and an internal expansion volume where the refrigeration occurs at the coldest end of each stage.
As a result of the development of these cryocoolers, small-scale (class S) liquefaction plants have become commercially available, however performance of these liquefiers is presently limited to less than 2 liters/day/kW. In these liquefiers, the gas to be liquefied does not undergo the complex thermodynamic cycles, but rather cools simply by thermal exchange with either the cold stages of the cryocooler, or with heat exchangers attached to the cold stages of the cryocooler. In these small-scale liquefiers, a cryocooler coldhead operates in the neck of a double-walled container, often called a Dewar, which contains only the gas to be liquefied and is thermally insulated to minimize the flow of heat from the outside to the inside of the container. After the gas condenses, the resulting liquid is stored inside the inner tank of the Dewar.
Ideally such small-scale liquefiers based on a cryocooler would achieve an efficiency comparable to that of the large and medium scale liquefiers. However, in practice, the achievable liquefaction performance in terms of liters per day per kW has been significantly less for these small-scale liquefiers than the performance realized by the larger Class M and Class L liquefaction plants. Accordingly, there is much room for improving the performance of small-scale liquefiers, and such improvements would be of particular benefit in the art.